Among the aforementioned substrates, there is, in particular, a substrate known under the acronym “SOI” (which stands for “Silicon On Insulator”), in which an insulator layer, generally an oxide layer, is inserted and buried between a surface silicon active layer and a support substrate.
The expression “active layer” denotes a layer of semiconductor material (of silicon in the case of an SOI), in which and/or on which components for electronics, optics and/or optoelectronics will be subsequently manufactured.
Such substrates are generally obtained by molecular bonding of a donor substrate made of silicon to a support substrate, one and/or the other of these two substrates being covered, prior to the bonding, with an insulator layer, then by detachment of the active silicon layer from the donor substrate along a weakened zone.
This weakened zone is, for example, obtained by implantation of atomic species, for example, according to the process known under the trademark SMARTCUT®. 
The SOI substrates thus obtained are generally subjected to finishing steps that aim to thin the thickness of the active silicon layer that was transferred to the support substrate.
Already known from the prior art is a thinning process known under the name “sacrificial oxidation/deoxidation,” which consists of carrying out a thermal oxidation of the surface portion of the active silicon layer, so as to form a layer of silicon oxide (SiO2), then in carrying out a deoxidation (removal) of this sacrificed oxide layer.
This thermal oxidation is generally carried out by subjecting the active silicon layer to a temperature between 700° C. and 1100° C. for a duration of a few minutes to a few hours, depending on the desired oxide thickness, in a conventional furnace for dry or wet oxidation.
The thickness of the thermal silicon oxide (SiO2) thus formed is unfortunately not uniform. This uniformity is also extremely difficult to control during the sacrificial oxidation step itself.
Consequently, after the removal of this thermal oxide layer (deoxidation), the residual thickness of the active silicon layer is no longer uniform. There is, therefore, a sizeable degradation of the active silicon layer between its step of transfer to the support and its finished state.
It would, therefore, be desirable to improve this thinning process.
This problem is faced even more during the production of a substrate known under the acronym “FDSOI” (which stands for “Fully Depleted Silicon On Insulator”), in which the surface active silicon layer has a thickness between approximately 5 and 30 nanometers, typically between 5 and 12 nm.
It is easily understood that a thickness irregularity in an extremely thin layer may be particularly problematic, since if the silicon layer obtained after the oxidation/deoxidation step is too thin or too thick at certain points, the electrical properties of the components that will be subsequently manufactured thereon risk being greatly degraded.
This problem is also faced whether the FDSOI substrate is of the type in which the thickness of the buried oxide layer is close to 150 nanometers or is of the type in which the thickness of this layer is less than or equal to 30 nanometers, as in the substrates known under the acronym “FDSOI UTBOX” (which stands for “Fully Depleted Silicon On Insulator Ultra Thin Buried Oxide”).
Among the electrical characteristics that may be greatly affected by variations of the thickness of the active silicon layer, is the threshold voltage of FDSOI transistors; that is to say, the voltage below which the transistor is said to be “OFF” and above which it is said to be “ON.” The sensitivity of the threshold voltage—to the thickness variations of the active layer is of the order of 25 mV/nm.
The continuous miniaturization of transistors leads to narrowing of the tolerance interval in which the threshold voltage of all the transistors of a single component must be found and, therefore, in reducing the tolerated variations of the thickness of the active layer.